Microelectromechanical or nanoelectromechanical system (MEMS or NEMS) devices range from very simple structures without moving elements or parts to quite complex systems with several moving elements or parts under control. MEMS devices include transducers such as microcantilevers, microsensors or microactuators, mechanically functional microstructures such as microfluidics such as valves, pumps or flow channels or microengines such as gears, turbines or combustion engines. MEMS devices find a variety of engineering applications including automotive systems, health care applications, automated manufacturing, instrumentation, environmental monitoring and control applications, consumer products or aerospace.
A MEMS cantilever comprises a base and a cantilever beam projecting from the base. Surface of the cantilever is covered with analyte molecules which when come in contact with receptor molecules generate surface stress on the cantilever and cause deflection of the cantilever. Deflection of the cantilever is a function of the receptor molecules and is measured either by electrical read out systems or optical read out systems such as AFM (Atomic Force Microscope). In AFM, microcantilever is used as an imaging probe in sensing various physical, chemical or biological elements. A laser light source is directed onto the cantilever and the reflected beam is sensed by position sensing diode. In electrical read out systems, deflection of the cantilever due to stress is measured as change of electrical property of the cantilever. Optical read out systems are heavy and hefty, complicated in construction and expensive as compared to electrical read out systems. Therefore, electrical read out systems are preferred for low cost sensing applications.
U.S. Pat. No. 8,278,725B2 describes a micromechanical structure and a method of fabricating the same. The micromechanical structure comprises a silicon substrate, a micromechanical element formed directly on the substrate and an undercut (recess) formed in the substrate underneath a released portion of the micromechanical element. U.S. Pat. No. 7,763,947B2 describes a piezothin-filmdiode (piezo-diode) cantilever microelectromechanical system (MEMS) and associated fabrication processes. Thin-films are deposited overlying a substrate which can be made of glass, polymer, quartz, metal foil, Si, sapphire, ceramic or compound semiconductor materials. Amorphous silicon (a-Si), polycrystalline Si (poly-Si), oxides, a-SiGe, poly-SiGe, metals, metal-containing compounds, nitrides, polymers, ceramic films, magnetic films and compound semiconductor materials are examples of thin-film materials. A cantilever beam is formed from the thin-films and a diode is embedded with the cantilever beam.
The diode is made from a thin-film shared in common with the cantilever beam. The shared thin-film may be a film overlying a cantilever beam top surface, a thin-film overlying a cantilever beam bottom surface or a thin-film embedded within the cantilever beam. In one aspect, a plasma enhanced chemical vapor deposition (PECVD) method is used to control the MEMS structure thickness and a pre-deposited sacrificial film is used to support the MEMS structure fabrication. The sacrificial film is removed to define an air gap between the MEMS structure and the substrate. The diode is a lateral PIN diode, having a serpentine pattern formed in the Si film layer overlying the cantilever beam top surface.
U.S. Pat. No. 6,720,267B1 describes a method for forming a micro-electromechanical system (MEMS) and more particularly to a method for forming a cantilever beam type MEMS applied in the field of fiber-optic communication. A semiconductor substrate comprising a heavily doped layer and a first dielectric layer formed within the semiconductor substrate is further formed with at least two first conductors connected to a surface of the heavily doped layer in the first dielectric layer. A second dielectric layer not connected to the surface of the heavily doped layer in the first dielectric layer is formed between the first conductors. A patterned sacrificial layer is formed on the semiconductor substrate that covers the second dielectric layer, the first dielectric layer and the first conductors.
A third dielectric layer is formed on the semiconductor substrate that covers the patterned sacrificial layer. A fourth dielectric layer not connected to a surface of the patterned sacrificial layer is formed in the third dielectric layer. At least two second conductors are formed on the third dielectric layer corresponding to the underlying first conductors formed on two sides of the second dielectric layer. The fourth dielectric layer is etched to form a plurality of openings in the fourth dielectric layer. A cap layer is formed on the semiconductor substrate to cover the second conductors, the fourth dielectric layer and the third dielectric layer. The patterned sacrificial layer is finally removed.
U.S. Pat. No. 6,156,216 describes a method for making nitride cantilevers devices for surface probing. A substrate with a top silicon working surface and a bottom silicon working surface is formed with a silicon stylus of a predetermined area with a base and an apex by etching the top surface of the substrate. A nitride layer of a predetermined thickness is deposited on the top silicon working surface and silicon stylus to produce a nitride covered working surface and a nitride covered silicon stylus. A resistive is spin coated on the nitride covered working surface. A predetermined area of the nitride covered silicon stylus is etched to expose apex of the silicon stylus.
U.S. Pat. No. 7,022,540B2 describes a fabrication method of a cantilever sensor by depositing a silicon nitride film respectively onto the top and the bottom surfaces of a silicon substrate. A silicon oxide film is deposited onto the top silicon nitride film. A lower electrode is deposited onto the silicon oxide film. A first piezoelectric film and a second piezoelectric film are deposited onto the lower electrode so as not to contact with each other. An upper electrode is deposited respectively onto the first and second piezoelectric films. A protecting film is deposited onto the silicon oxide film, the lower electrode, the first and second piezoelectric films and the upper electrodes.
A first opening and a second opening are formed on the protecting film of the upper electrode and on the protecting film of the lower electrode, respectively. A first and a second contact pads respectively formed on the first and second openings. Part of the silicon nitride film on the bottom surface of the substrate is removed and a membrane of predetermined thickness is formed by etching the silicon substrate in which the silicon nitride film is removed. A cantilever is formed by removing part of the membrane by etching the surrounding portion of the cantilever.
U.S. Pat. No. 8,393,011B2 describes MEMS devices comprising at least one cantilever and at least one piezoresistor. The cantilever comprises silicon nitride or silicon carbide and the piezo resistor is doped silicon. The MEMS devices are used for height sensing. U.S. Pat. No. 6,886,395B2 describes a method of making a probe having a cantilever and a tip. A substrate having a surface and a tip extending substantially orthogonally from the surface is provided. An etch stop layer is deposited on the substrate to protect the tip followed by a silicon nitride layer on the etch stop layer. An etch operation is used to release the cantilever and expose the etch stop layer protecting the tip. Preferably, the tip is silicon and the cantilever supporting the tip is silicon nitride. U.S. Pat. No. 5,399,232A describes a dielectric cantilever arm stylus with an integrally formed pyramidal tip. The tip is molded in a pyramidal pit etched in a later-removed silicon substrate. An integrally-formed cantilever arm is also formed as the tip is being formed. Various thin film materials form the cantilever arm and the tip. The dielectric is silicon nitride and cantilever arm is anodically bonded to a glass block.
U.S. Pat. No. 5,066,358 describes a method of forming a nitride cantilever with an integral conical silicon tip at the free end thereof. A top layer of silicon dioxide is patterned into a tip mask on a doped or epitaxial silicon layer in a silicon substrate. Photoresist is spun on the silicon substrate and patterned and the silicon is etched to define a cantilever pattern in the substrate with the tip mask positioned to be near the free end of a nitride cantilever to be subsequently formed. A bottom layer of silicon dioxide is formed on the silicon substrate and then patterned and etched to define a masking aperture on the bottom silicon dioxide layer. The bottom of the silicon substrate is anisotropically etched through the masking aperture and the etch stops at the doped silicon layer.
Alternatively, electrochemical etching is done by applying an electric potential across the P-N junction between the doped silicon layer and the appropriately-doped substrate. This releases the free end of the doped silicon layer from the silicon substrate. The anisotropic etch preferentially etches all of the crystal planes of the silicon substrate except the planes to leave a silicon base from which extends the silicon surface layer as a cantilever. A nitride layer is then formed on the silicon substrate and dry etched from the top surface of the doped silicon surface layer to form a nitride cantilever on the bottom of the silicon substrate. The doped silicon layer is etched away while the tip mask helps to form a pointed silicon tip near the free end of the nitride cantilever.
Nitride piezoresistive cantilever comprising a silicon substrate and a nitride layer deposited on the substrate is reported. Nitride is deposited on the substrate by LPCVD (Low-Pressure Chemical Vapor Deposition) technique and the substrate is etched by KOH (potassium hydroxide) to form the cantilever. (Optimised cantilever biosensor with piezoresistive read-out P A Rasmussena, J Thaysenb, O Hansena, S C Eriksena, A Boisen). Low stress nitride cantilever with platinum piezoresistor is also reported. The silicon substrate is etched to form cantilevers. (Gas Flow Sensing with a Piezoresistive Micro-Cantilever Yu-Hsiang Wang, Chia-Yen Lee Rong-Hua Ma Lung-Ming Fu-Taiwan).
US 2006/0075803A1 describes a polymer based cantilever array for use as a sensor comprising a platform and a multitude of polymer-based cantilevers attached to the platform. Each of the cantilevers is coupled to an optical sensing means adapted to sense deformations of an individual cantilever. The cantilevers may be coated with a first layer of metal such as gold and/or a second layer being a molecular layer capable of functioning as a receptor layer for molecular recognition. The cantilever array is fabricated based on photolithography or micromoulding. U.S. Pat. No. 8,122,761B2 describes another polymer based microcantilever sensor including a supporting substrate, a cantilever spring element at least partially disposed over the support substrate, a probe layer disposed over the first side of the cantilever spring element and a piezoresistive transducer attached to the second side of the cantilever spring element. The cantilever spring element is characterized by having a first side and a second side and comprising a polymer having a Young's modulus less than about 100 Gpa.
U.S. Pat. No. 6,136,208 describes a method of manufacturing a planar microprobe comprising a upper cantilever beam including a first electrode, a supporting pad coupled to the upper cantilever beam, and a lower cantilever beam coupled to the supporting pad, situated below the upper cantilever beam and spaced by a distance from the upper cantilever beam. Besides, the lower cantilever beam comprises a second electrode in cooperation with the first electrode to control a vertical displacement of the lower cantilever beam by applying an external voltage thereto. A tip is coupled to the second electrode. The microprobe is manufactured by depositing a photoresist sacrificial layer on a lower cantilever beam. Metal is electroplated to form an upper cantilever beam. Then, residual silicon, silicon oxide and photoresist are etched to release the microprobe.
Cobalt-nickel microcantilevers for biosensing are also reported. Cobalt nickel cantilevers are deposited on copper sacrificial layer and SU8 polymer is used to anchor the cantilevers (Olgaç Ergeneman, Marcel Suter, George Chatzipirpiridis, Kartik M Sivaraman, Patric Eberle, Salvador Pané, Eva Pellicer, Jordi Sort and Bradley J Nelson—Journal of Intelligent Material Systems and Structures published online 17 Oct. 2012).
MEMS devices and fabrication methods as discussed above suffer from one or more of the following problems or disadvantages. Non-polymer cantilever devices are generally expensive and are also complex and complicated in construction. Fabrication methods of the devices involve a large number of steps and are time consuming and cumbersome. Fabrication methods also involve expensive procedures like wafer bonding or DRIE (deep reactive ion etching) or TMAH (tetramethylammonium hydroxide) wet etching and are unsuitable for low cost applications. Substrate either gets consumed or forms a part of the devices. Polymer cantilevers are not stable in liquid medium. There is thus need for MEMS cantilevers which are simple in construction, stable under test conditions like liquid, vapour or gas, robust and cost effective and for methods of fabrication of such cantilevers, which are simple and easy and convenient to be carried out, less time consuming and cost effective.